Downhole gas compression

ABSTRACT

A downhole gas compression system is adapted for location in a bore of a natural gas-producing well ( 10 ), the system comprising an axial flow compressor ( 32 ) and a gas-filled electric drive motor ( 30 ). The motor drives the compressor to compress the produced gas, the compressed gas being directed upwardly through production tubing ( 20 ) to surface.

FIELD OF THE INVENTION

[0001] This invention relates to downhole gas compression, and inparticular to the provision of a gas compression system suitable for usein downhole applications, and having utility in facilitating recovery ofnatural gas from subsurface hydrocarbon-bearing formations.

BACKGROUND OF THE INVENTION

[0002] In oil and gas production operations, a drilled bore extends fromsurface to intersect a hydrocarbon-bearing formation. The hydrocarbonmay be in the form of a liquid or gas, or a mixture of both; forbrevity, reference will be made primarily herein to production of gas.Initially, the gas, known as the produced gas, is often at sufficientpressure that it will flow from the formation, through the well bore, tosurface. As the gas travels up through the bore the gas cools, and thegas velocity must be sufficient to carry the resulting condensates tosurface. However, when a well has been producing gas for some time andthe volume of gas remaining in the formation has decreased, oftenreferred to as a depleting gas well, the formation pressure may fallbelow the wellhead manifold pressure, or the difference between thereservoir pressure and wellhead pressure may be such that a satisfactoryflow rate from the well cannot be maintained; the gas must then bepumped out of the well. This is most effectively achieved by compressingthe gas at a point in the well, preferably close to the productionformation. However, there are many difficulties associated withcompressing gas in the well, some related to the restricted spaceavailable in the well to accommodate the compressor, and also thedifficulty in supplying power to the compressor.

[0003] To achieve the pressures sought in the space available, it isgenerally considered necessary to utilise a high speed compressor. WO97/33070 (Shell Internationale Research Maatschappij B.V.) describes adownhole multistage rotary compressor driven by a brushless permanentmagnet motor and described as being capable of operating at a speedabove 5000 rpm. To reduce friction within the compressor, the compressorshaft journal bearings are gas lubricated, the gas being the producedgas which is supplied to the bearings via a small auxiliary compressorunit mounted to the main compressor. The motor and optional gearbox musthowever be liquid cooled and lubricated, and are therefor located inappropriate liquid-filled chambers isolated from the compressor byconventional seals.

[0004] It is among the objectives of embodiments of the presentinvention to provide a downhole compression system which provides animproved performance over existing proposals.

SUMMARY OF THE INVENTION

[0005] According to a first aspect of the present invention there isprovided a downhole gas compression system adapted for location in abore, the system comprising an axial flow compressor and a gas-filledelectric drive motor.

[0006] The invention also relates to a method of compressing gasdownhole, utilising a compressor driven by a gas-filled electric motor.

[0007] The use of a gas-filled motor avoids the friction lossesassociated with conventional oil-filled motors; friction losses in therotor/stator gap and churning losses in oil-filled motors placerestrictions on the speeds such motors may achieve while containinglosses within tolerable levels.

[0008] The gas utilised to fill the motor may vent into the well bore,and join the produced fluid, preferably via gas valves which operate asgas seals in the opposite flow direction, preventing ingress of wellfluids to the motor in the event of loss of supply gas pressure.

[0009] Conveniently, the motor and compressor are substantially axiallyaligned within an elongate housing, such that they may be accommodatedin the confines of a well bore.

[0010] Preferably, the motor is gas lubricated, with gas being suppliedto the motor bearings, which bearings are preferably hydrodynamic, butmay alternatively be hydrostatic.

[0011] Preferably both the compressor and motor are liquid free, thatis, the compressor set does not contain any liquids such as water,liquid hydrocarbons, liquid lubricants and the like.

[0012] Preferably, the motor is also gas cooled. In one embodiment, thisallows use of produced gas to cool the motor, which gas may be directedover or around the motor as appropriate, such that the motor does nothave to be contained within a finite volume of liquid, typically alubricating oil, held in a fluid-tight housing; as described in WO97/33070, this conventional arrangement places restrictions on theenergy which may be added to the gas, as the compressed gas must bemaintained at a temperature low enough to permit cooling of the oil andto avoid a phase change of the liquid motor lubricants.

[0013] Preferably, the motor drives the compressor directly, preferablyon a single shaft, such that there is no requirement for a gearboxrequiring liquid lubrication and cooling, and thus high speed shaftsealing arrangements.

[0014] Preferably, the motor is a brushless permanent magnet motor, andthus typically of relatively high efficiency, and most preferably of oneor both of high electrical frequency and variable speed. Such a motor,if gas filled and gas lubricated, may be driven at high speeds,typically between 20,000 and 70,000 rpm; the optimum speed will dependon a number of factors, including the available bore diameter, thelocation of the compressor in the bore, and the properties of theproduced gas. The motor may be powered by electrical supply fromsurface, via an inverter.

[0015] In one embodiment, a plurality of motors and compressors areprovided; the compressors may be mounted in series and the motors may beconnected in parallel. A motor controller and inverter may be mounted atsurface, power distribution to the motors being such that the group ofmotors operates effectively as a single machine. Alternatively, aplurality of inverters are installed downhole, one for each motor, suchthat each motor can be controlled separately of the others. Thisarrangement provides added flexibility in operation, or redundancy, tosuit changing well bore flowing conditions.

[0016] Preferably, the compressor is gas lubricated, gas being suppliedto the compressor bearings, which are preferably hydrodynamic.Alternatively, the bearings may be hydrostatic, however such bearingstend to require a greater gas supply.

[0017] Preferably, gas is supplied to one or both of the motor andcompressor from surface, and is preferably clean and liquid freeproduced gas, or other gas which is compatible with the produced gas.The gas may be compressed at surface by an auxiliary compressor.Alternatively, produced gas from the well bore may be utilised.Preferably, this gas is obtained at compressor discharge and is passedthrough a downhole solids and entrained liquid separator and anauxiliary compression stage before being passed to one or both of themotor and compressor.

[0018] The compressor may be single or multistage.

[0019] In some applications, where liquid slug flow may occur and whichwould be detrimental to compressor performance, a liquid separator maybe provided before the compressor inlet. Most preferably, the separatedliquid is driven, preferably by gravity, back into a section of theformation which is isolated from the production zone. Most convenientlya centrifugal separator, such as a cyclone, is utilised.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other aspects of the invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

[0021]FIG. 1 is a diagrammatic illustration of a downhole gascompression system in accordance with a preferred embodiment of thepresent invention;

[0022]FIG. 2 is an enlarged cross-sectional view of the compressor andmotor of the system of FIG. 1;

[0023]FIG. 3 is a diagrammatic illustration of a downhole gascompression system in accordance with a second embodiment of the presentinvention; and

[0024]FIG. 4 is a cross-sectional view of part of a downhole gascompression system in accordance with third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] Reference is first made to FIG. 1 of the drawings, which is adiagrammatic illustration of a downhole gas compression system inaccordance with a preferred embodiment of the present invention. Thesystem is installed in a depleting gas well 10, the well comprising abore 12 extending from the surface to a gas producing formation. Most ofthe length of the bore 12 is lined with metal casing 14, while the lowerend of the bore 12, which intersects the gas producing formation, islined with selectively perforated metal liner 16, the liner 16 beingsupported from and sealed to the casing 14 by an appropriate hanger 18.Within the casing 14, a smaller diameter string of production tubing 20is utilised to transport the gas to surface. The upper end of the tubing20 is secured and sealed to the casing 14 by a tubing hanger 22, and theannulus 24 between the casing 14 and the production tubing 20 is sealedby a packer 26. A safety valve 28 is provided within the productiontubing 20, and mounted towards the lower end of the tubing 20 is anelectric motor 30 and an axial flow compressor 32. A motor controller34, incorporating an inverter, is provided on surface and provides powerto the motor 30 via a cable 36, which passes through the annulus 24. Afurther cable 37 carries signals from the motor and compressor tofacilitate monitoring thereof. The motor 30 drives the compressor 32 tocompress produced gas, the compressed gas being directed upwardlythrough the production tubing 20 to surface. At surface, a proportion ofthe produced gas is diverted into a solid and liquid separator 38, theresulting liquid free clean gas being then passed though a filter 40 andauxiliary compressor 42 before being passed down through the annulus 24,in coiled tubing 44, to the motor 30 and compressor 32, where the gas isutilised to lubricate the motor 30 and the compressor 32, as describedbelow.

[0026] Reference is now also made to FIG. 2 of the drawings, which is anenlarged cross-sectional view of the compressor 32 and motor 30. Thecompressor 32 is of the multi-stage centrifugal axial flow type and iscoupled, via an inlet connector 46, to tubing 48 in fluid communicationwith the gas producing formation, via the perforated liner 16. The gaspasses up through the compressor 32 and is then directed round the motorcasing 50, before passing through a discharge connector 52 and into theproduction tubing 20.

[0027] The motor 30 is a variable speed permanent magnet motor anddrives the compressor 32 directly, via a combined motor\compressor shaft54. The motor 30 is cooled by the flow of produced gas over the motorcasing 50 and is gas filled. Further, both the motor 30 and thecompressor 32 are gas lubricated, as described below.

[0028] The illustrated motor and compressor set comprises two motorjournal bearings 56, 57, a double action thrust bearing 58, and threecompressor journal bearings, 59, 60, 61 (in short compressor sets withfew stages (one or two), the compressor stages may be overhung from themotor, this arrangement requiring no additional journal bearings in thecompressor). All of the bearings 56-61 are hydrodynamic and are eachsupplied with filtered dry clean produced gas from surface, via thecoiled tubing 44. The bearing gas lubricant, which also serves as themotor fill gas, vents into the tubing 20, and joins the produced fluid,via gas valves 62 which operate as gas seals in the opposite flowdirection, thus preventing ingress of produced fluids to the bearings ormotor in the event of loss of supply gas pressure.

[0029] It will be apparent to those of skill in the art that the use ofa gas filled and cooled variable speed permanent magnet motor 30 asdirect drive for a gas lubricated axial flow compressor 32 allow thecompressor to run at very high speeds, in the region of 20,000 rpm to70,000 rpm, allowing the produced gas to be pressurised to a level whichallows efficient extraction of gas from depleted wells.

[0030] Reference is now made to FIG. 3 of the drawings, whichillustrates a downhole gas compression system in accordance with asecond embodiment of the present invention, the system being adapted forapplications in which liquid slug flow may occur, and which flowconditions would be detrimental to compressor performance. The majorityof the features of the system are the same as those illustrated anddescribed with reference to FIGS. 1 and 2; these features will not bedescribed again in any detail, and bear reference numerals correspondingto the numerals used in FIGS. 1 and 2, prefixed with a “1”.

[0031] The perforated liner 116 which intersects the productionformation also extends into a lower liquid re-injection zone, where theliner 116 is also perforated.

[0032] Gas and liquid pass from the production zone into the upperportion of the liner 116, and then upwardly into a gas and liquidcyclone separator 70, the produced gas passing upwardly through thecompressor inlet tubing 148 to the compressor 132, while the separatedproduced liquid passes downwardly, relying on natural gravity, throughliquid return tubing 72. The liquid return tubing 72 carries the liquidinto the lower portion of the liner 116, isolated from the upperproducing portion by a packer 74, where the separated liquid isre-injected into the formation. Thus, the gas reaching the compressor132 is substantially liquid free.

[0033] Reference is now made to FIG. 4 of the drawings, which is across-sectional view of part of a downhole gas compression system inaccordance with a third embodiment of the present invention. In thisembodiment, multiple motors\compressor sets are provided, thecompressors 232 a, 232 b, 232 c being mounted in series, while themotors 230 a, 230 b, 230 c are connected in parallel. As with theabove-described first and second embodiments, clean gas is suppled fromthe surface to the motor and compressor bearings, and the motors arecooled by the flow of produced gas over the motor casings.

[0034] As with the first described embodiment, the motor controller andinverter may be provided at the surface, power distribution to theindividual motors downhole being such that the multiple motors operateeffectively as a single machine. Alternatively, the inverters may beinstalled downhole, one for each motor, such that each motor can becontrolled separately of the others. This arrangement provides an addeddegree of flexibility in operation and\or redundancy, to suit changingwell bore flowing conditions.

[0035] It will be apparent to those of skill in the art that theabove-described embodiments are merely exemplary of the presentinvention, and that various modifications and improvements may be madethereto, without departing from the present invention. For example,rather than providing gas to lubricate the motor and compressor bearingsand fill the motor from surface, the gas may be taken from thecompressor discharge, solids and liquids being removed by separationdownhole by cyclones or other arrangements, and after furthercompression in an auxiliary compressor stage the gas being fed to thebearings and motor. Further, the illustrated embodiments show the motormounted above the compressor, however in other embodiments thecompressor may be mounted above the motor, this offering the advantagethat the produced gas in contact with the motor casing, and acting tocool the motor, is likely to be at a lower temperature than thecompressed produced gas flowing from the compressor outlet.

I claim:
 1. A downhole gas compression system adapted for location in abore, the system comprising an axial flow compressor operativelyassociated with a gas-filled electric drive motor.
 2. The system ofclaim 1, further comprising gas valves for permitting gas utilised tofill the motor to vent from the motor and into a bore in which the motoris located.
 3. The system of claim 2, wherein the gas valves are adaptedto operate as gas seals in the opposite flow direction to the ventdirection.
 4. The system of claim 1, wherein the drive motor is gaslubricated.
 5. The system of claim 1, wherein the motor comprises gaslubricated bearings.
 6. The system of claim 5, wherein at least some ofthe motor bearings are hydrodynamic.
 7. The system of claim 5, whereinat least some of the motor bearings are hydrostatic.
 8. The system ofclaim 5, wherein the motor includes a gas supported thrust bearing. 9.The system of claim 8, wherein a gas supported thrust bearing isprovided between the motor and the compressor.
 10. The system of claim1, wherein the motor is gas cooled.
 11. The system of claim 10, whereinthe motor is adapted to be cooled by produced gas.
 12. The system ofclaim 1, wherein the motor is adapted to drive the compressor directly.13. The system of claim 12, wherein the motor drives the compressor on asingle shaft.
 14. The system of claim 1, wherein the motor is apermanent magnet motor.
 15. The system of claim 14, wherein the motor isof high electrical frequency.
 16. The system of claim 14, wherein themotor is operable at variable speed.
 17. The system of claim 1, whereinthe motor is adapted to be driven at speeds of between 20,000 and 70,000rpm.
 18. The system of claim 1, wherein the motor is adapted to bepowered by electrical supply from surface, via an inverter.
 19. Thesystem of claim 1, wherein a plurality of motors and compressors areprovided.
 20. The system of claim 19, wherein the compressors aremounted in series.
 21. The system of claim 19, wherein the motors areconnected in parallel.
 22. The system of claim 19, further comprising amotor controller and inverter adapted to be mounted at surface, powerdistribution to the motors being such that the group of motors willoperate effectively as a single machine.
 23. The system of claim 19,further comprising a plurality of inverters adapted to be installeddownhole, one for each motor, such that each motor can be controlledseparately of the others.
 24. The system of claim 1, wherein thecompressor comprises gas lubricated bearings.
 25. The system of claim24, wherein the compressor bearings are hydrodynamic.
 26. The system ofclaim 24, wherein the compressor bearings are hydrostatic.
 27. Thesystem of claim 1, wherein means is provided for supplying gas to atleast one of the motor and compressor.
 28. The system of claim 27,wherein said means is adapted to provide clean and liquid free producedgas.
 29. The system of claim 28, wherein said means includes means forremoving at least one of solids and liquids from the gas.
 30. The systemof claim 27, wherein an auxiliary compressor is provided to compress thegas.
 31. The system of claim 27, wherein said means is adapted to supplygas from surface.
 32. The system of claim 1, wherein means is providedfor supply produced gas directly from a bore in which the motor andcompressor are located to at least one of the motor and compressor. 33.The system of claim 32, further comprising a downhole solids andentrained liquid separator and an auxiliary compression stage whereby,in use, gas obtained at compressor discharge is passed therethroughbefore being passed to one or both of the motor and compressor.
 34. Thesystem of claim 1, wherein the compressor has a single stage.
 35. Thesystem of claim 1, wherein the compressor is multistage.
 36. The systemof claim 1, wherein the compressor comprises an inlet and a liquidseparator is provided before the compressor inlet.
 37. The system ofclaim 36, wherein, in use, separated liquid is driven back into asection of formation isolated from the production zone.
 38. The systemof claim 36, wherein a centrifugal separator is provided.
 39. The systemof claim 1 wherein the compressor is liquid free.
 40. The system ofclaim 1 wherein the motor is liquid free.
 41. The system of claim 1wherein the compressor and motor are axially aligned and areaccommodated in an elongate housing.
 42. A downhole gas compressionsystem for location in a bore, the system comprising an axial flowcompressor and a gas-filled permanent magnet electric drive motor.
 43. Amethod of compressing gas in a bore, utilising a compressor driven by agas-filled electric motor.
 44. A downhole gas compression systemcomprising an axial flow, gas lubricated, liquid free compressordirectly driven by a gas filled, gas cooled, gas lubricated, liquidfree, variable speed, permanent magnet electric drive motor.
 45. Thesystem of claim 44 wherein the compressor and motor are axially alignedand are accommodated in an elongate housing.
 46. A method of compressinggas in a bore for transporting the gas to surface, the methodcomprising: passing produced gas through a downhole, axial flow, gaslubricated, liquid free compressor; and directly driving the compressorwith a downhole, gas filled, gas cooled, gas lubricated, liquid free,variable speed, permanent magnet electric drive motor.
 47. The method ofclaim 46, wherein filtered dry lubricating gas is supplied to lubricatethe compressor and motor.
 48. The method of claim 46, wherein thelubricating gas is supplied from surface.